1. Field of the Invention
This invention relates to solid ion conducting materials, capable of being used as electrolytes.
More specifically, it relates to materials made from solid solutions of polymers and of alkali metal salts, such as lithium salts, possessing a sufficient ion conductivity for them to be used as electrolyte in electrochromic glass panes, that is in panes in which the light transmission can be modified by the action of a potential difference as a result of the presence of a film of an electrochromic material, such as tungsten oxide (WO.sub.3), having the particular feature of changing color under the effect of a reversible insertion of cations.
2. Discussion of the Background
It is now well known to associate salts of alkali metals with a basic polymer matrix having heteroatoms like oxygen, such as polyethylene oxide (PEO) or nitrogen, such as polyethylene imine (PEI). The ion conduction of the material thus formed is obtained by dissociation of the salt, the cation being solvated by means of free electronic doublets of the polymer heteroatoms, the anion according to certain authors being solvated by the polymer via hydrogen bonds. (M. B. Armand, Annual Review of Materials Science, 1986,16,245:261).
Until now, the majority of studies have been directed to electrolytes based on polyethylene oxide (PEO), which have the advantage of producing relatively high ion conductivities. Nevertheless, this criterion of ion conductivity is not the only one to be taken into account in the choice of a macromolecular material.
It is, for example, desirable for the polymer to be conductive at the operating temperatures envisaged, which in the case of an electrochromic system assumes good conductivity at ambient temperature. It is known that the polymers studied are conducting only if they have an amorphous structure, that is a disorganized structure, which is the one most suitable for dissociating and solvating the salt. Any initiation of crystallization causes the cohesion energy of the polymer to increase and has adverse effects upon the ion conductivity of the material.
Such an amorphous structure can only be obtained by operating at a temperature higher than the glass transition temperature Tg, which should therefore be as low as possible. Furthermore, in addition to amorphous structure, elastic behavior is helpful to ion conductivity because it favors the flexibility and mobility of the polymer segments. It also facilitates the use of the final material. It may be noted that this characteristic is difficult to obtain as the value of Tg varies as a function of the quantity of salt solvated in the polymer, and should therefore remain at a low level with the usual concentrations of added salt.
On the other band, independently of the operating temperature, it is desirable for the polymer to possess, at ambient temperature, sufficient plasticity for it to be incorporated as a thin film with good bonding to the other layers of the system.
In the case of the PEO, an ion conductivity of 10.sup.-5 ohm.sup.-1.cm.sup.-1, a value usually considered to be the minimum to give the material an electrolyte function, is not obtained unless the material is raised to a temperature higher than 80.degree. C., as taught in FR 2,442,514. However, although it may be possible to heat the material to temperatures above 80.degree. C. for applications in electrochemical generators, the same is not generally true in the case of electrochromic glass panes, where the other films of the system may be damaged. To subject a polymer film to such high temperatures can lead to accelerated aging and degradation of its mechanical properties.
In the case of PEI, it has been proposed (C. S. Harris, Macromolecules, 1987, 20:1778-1781), for the purpose of improving the amorphous and flexible character of the polyethylene imine (PEI), to not use linear PEI, but branched PEI (BPEI), the latter having a certain relative proportion of primary, secondary and tertiary nitrogen atoms. However, in order to achieve conductivity values higher than or equal to 10.sup.-5 ohm.sup.-1.cm.sup.-1, it is necessary to raise this polymer to 90.degree. C., a temperature which a priori is too high both for the polymer itself and for the other films of the electrochromic system. Furthermore, the investigation was limited to the cation Na.sup.+, whereas in the present state of the art the electrochromic systems are based upon the reversible insertion, by certain materials, either of protons or of lithium cations (Li.sup.+), and not of sodium cations (Na.sup.+).
A need continues to exist for improved solid ion conducting materials capable of functioning as electrolytes.